The present invention is directed to optical communications. More particularly, the present invention is directed to an optical dispersion compensator.
The characteristics of an optical fiber affect the nature of a signal pulse as it traverses the fiber. A major concern, especially in long-haul optical networks, is optical pulse dispersion. Optical pulse dispersion in an optical communication network causes optical signal pulses to spread out in the time domain.
Optical pulse dispersion is primarily caused by the differences of the propagation velocity of wavelength components, optical modes, or polarization comprising the optical pulse. Dispersion leads to intersymbol interference in which the optical pulses spread out and overlap one another, thus making it impossible for the detection of the individual pulses. In order to counter the effects of intersymbol interference it is required that the pulses be spaced further apart, which directly limits the possible data rate.
One method of avoiding optical dispersion is by using dispersion shifted fiber that prevents dispersion at a certain wavelength. For example, most dispersion shifted fiber is designed with a dispersion zero point around 1550 nm. However, the main drawback with dispersion shifted fiber is that it cannot be used where the fiber has already been installed. Digging up a few hundred kilometers of roadway to replace fiber types is extremely costly. Further, if wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d) is being used, the problems of four-wave mixing effectively prohibit the use of dispersion shifted fiber.
Various other methods have been proposed and implemented to counter the effects of optical dispersion, thus allowing for higher data rates than otherwise allowed. These methods typically provide for dispersion compensators to be inserted at intervals along a fiber. Dispersion compensators cancel the pulse dispersion that has occurred.
One type of dispersion compensator is dispersion compensating fiber. Dispersion compensating fiber has its core profile controlled to counteract dispersion. For example, in order to equalize an installed fiber link with dispersion at 1550 nm of 17 ps/nm/km (standard fiber) a shorter length of compensating fiber can be placed in series with it. The compensating fiber typically has a dispersion of xe2x88x92100 ps/nm/km in the 1550 nm wavelength band. Because the dispersion acts in the opposite direction to the dispersion of the standard fiber the compensating fiber xe2x80x9cundispersesxe2x80x9d the signal. Therefore, a 100 km length of standard fiber for operation at 1550 nm can be compensated by connecting it to 17 km of compensating fiber.
However, in most circumstances the existing optical fiber has already been installed, so the added length of fiber sits at one end of the link on a drum. This adds to attenuation and additional amplification may be needed to compensate for the compensating fiber. Compensating fiber has typical attenuation of 0.5 dB/km. In addition, the narrow core of dispersion compensating fiber makes it more susceptible to non-linear high power effects than standard fiber and it is also polarization sensitive.
Another method to reduce optical dispersion is mid-span spectral inversion, which requires inserting a device in the middle of the optical link to invert the spectrum. This process changes the short wavelengths to long ones and the long wavelengths to short ones. If the spectrum is inverted in the middle of the link (using standard fiber) the second half of the link acts in the opposite direction. When the optical pulse arrives, it has been rebuilt exactly compensated for by the second half of the fiber.
One problem with mid-span spectral inversion is that it is difficult to implement in all situations because an active device has to be placed into the middle of the fiber link. This may or may not be practical. In addition, mid-span spectral inversion xe2x80x9cundoesxe2x80x9d the effect of stimulated Raman scattering in WDM links, thus causing amplification problems.
Another know method for reducing optical dispersion is the use of chirped Fiber Bragg Gratings. In a chirped Fiber Bragg Grating, the spacing of the lines on the gratings vary continuously over a small range. Shorter wavelength light entering the grating travels along it almost to the end before being reflected. Longer wavelength light is reflected close to the start of the grating. Therefore, short wavelengths are delayed in relation to longer ones. Since the pulse has been dispersed such that short wavelengths arrive before the long ones, the grating can restore the original pulse shape and undo the effects of dispersion.
However, chirped Fiber Bragg Gratings need to be quite long. For single-channel application, up to 20 cm is commonly required. In a WDM system a fully continuous chirp would require a very long grating. To compensate for 100 km of standard (17 ps/nm/km) fiber the chirped grating needs to be 17 cm long for every nm of signal bandwidth. Therefore, a WDM system with channels spread over 20 nm would need a chirped Fiber Bragg Grating 340 cm long. Long Fiber Bragg Gratings are very difficult to construct.
Another problem with chirped Fiber Bragg Gratings is that they have a ripple characteristic in the Group Velocity Dispersion they produce. This ripple can be a source of transmission system noise. The longer the grating the larger the problem with ripple and its resultant noise. In addition, short Fiber Bragg Gratings are filters. When a signal is processed through many stages of filtering, the signal gets very narrow and distorted, and can also have increased noise.
Based on the foregoing, there is a need for an improved optical dispersion compensator.
One embodiment of the present invention is an optical dispersion compensator that includes a saturable absorber. Coupled to the saturable absorber is a pre-amplifier and a post-amplifier.